1. Field of the Invention
The present invention relates to a very small spot-size light beam forming apparatus for use in an optical information storage apparatus for storing information in an optical information storage disk in high density.
2. Description of the Background Art
In response to increase of density in which information is stored in an optical information storage disk, a very small spot-size light beam forming apparatus (optical recording head) for storing information with a resolution greater than light diffraction limit is suggested. Some of the apparatuses use scanning near field optical microscopes to which the principle of a probe microscope is applied. The scanning near field optical microscope reduces the diameter of an end portion (opening) of an optical waveguide such as an optical fiber so as to be sufficiently smaller than a wavelength of a propagating light such that a near field light is generated in a range having a micro diameter, which is smaller than the wavelength of the propagating light. Thus, the scanning near field optical microscope can be used as a very small spot-size light beam forming apparatus for storing information with a resolution greater than a light diffraction limit.
However, as described above, a diameter of an opening of an optical fiber or the like is sufficiently smaller (for example, smaller than or equal to 100 nm) than a wavelength of a propagating light, and therefore an amount of light applied to an optical information storage disk is reduced. Therefore, there is a problem (hereinafter, referred to as Problem 1) that it is difficult to preferably obtain a recording mark on the optical information storage disk. Further, a near field light generated at an opening of an optical fiber or the like has characteristic that the further the near field light goes from the opening, the more abruptly the near field light attenuates. Therefore, there is a problem (hereinafter, referred to as Problem 2) that, in order to obtain a recording mark on an optical information storage disk, a distance between an end portion (an opening at which the near field light is generated) of the very small spot-size light beam forming apparatus and a recording surface of the information storage disk needs to be always about λ/4 (λ represents a wavelength of a light propagating through an optical fiber or the like), which represents a substantially micro distance.
A very small spot-size light beam forming apparatus for solving Problem 1 is disclosed in Japanese Laid-open Patent Publication No. 2001-184702 (hereinafter, simply referred to as Document 1). The very small spot-size light beam forming apparatus disclosed in Document 1 includes a plurality of openings, and applies, through the plurality of openings, lights for recording to a same one track of an information storage disk multiple times. Thus, the very small spot-size light beam forming apparatus disclosed in Document 1 is allowed to preferably obtain a recoding mark on the optical information storage disk by applying a sufficient amount of light.
A very small spot-size light beam forming apparatus for solving Problem 2 is disclosed in Document 2. The very small spot-size light beam forming apparatus disclosed in Document 2 uses, as a lens, a material (refer to Document 4 and Document 5) which has a negative refractive index and exerts a perfect lens effect (refer to Document 3) that images (point light source) of incident lights which are not influenced by a light diffraction limit can be collected as a perfect same one light image (point light source). Therefore, in the very small spot-size light beam forming apparatus disclosed in Document 2, a distance between an end portion (end portion of a lens having a negative-refractive-index) of the apparatus and a recording surface of an information storage disk can be almost the same as a wavelength λ of a light propagating through an optical fiber or the like, thereby facilitating a control. According to Document 2, a solid immersion lens is used so as to form a point light source.    Document 1: Japanese Laid-Open Patent Publication No. 2001-184702    Document 2: “Near-field optical storage system using a solid immersion lens with a left-handed material slab”, Opt. Exp., Vol. 12, No. 20, pp. 4835 to 4840 (2004) FIG. 1    Document 3; J. B. Pendry, “Negative Refraction Makes a Perfect Lens”, Phys. Rev. Lett. 85, 3966 to 3969 (2000)    Document 4: R. A. Shelby, D. R. Smith, S. C. Nemat-Nasser, and S. Schultz, “Microwave transmission through a two-dimensional, isotropic, left-handed metamaterial” Appl. Phys. Lett., Vol. 78, No. 4, pp 489 to 491 (2001)    Document 5: Chiyan Luo, Steven G. Johnson, J. D. Joannopoulos, and J. B. Pendry, “All-angle negative refraction without negative effective index” Phys. Rev. B, Vol. 65, 201104 (R) (2002)
However, the very small spot-size light beam forming apparatus disclosed in Document 1 applies lights for recording to a same one track of an optical information storage disk multiple times, thereby causing another problem (hereinafter, referred to as Problem 3) that a rate at which information is recorded is reduced. Further, simple combination of the technique disclosed in Document 1 with the technique disclosed in Document 2 does not lead to solution of Problem 3.